Just taken on a site that has a rewrite rule in place to remove & redirect .php extensions. i.e /about.php will become /about.However, there are other rules in place and I have just noticed that if the url parameter doesn't match the rewrite condition a 501 Internal Server Error is thrown as opposed to a 404.It is definately the rule that removes the .php that is causing the issue as commenting it out solves the issue, however this rule must be left in place.